Full Idea
Williamson's view on modality is that everything that could exist does exist: since there could exist a talking donkey there actually exists some thing that could be a talking donkey.
Gist of Idea
If talking donkeys are possible, something exists which could be a talking donkey
Source
report of Timothy Williamson (Modal Logic as Metaphysics [2013], n20) by Ross P. Cameron - Truthmaking for Presentists n20
Book Reference
'Oxford Studies in Metaphysics Vol.6', ed/tr. Zimmerman,D/Bennett,K [OUP 2011], p.68
A Reaction
Well that thing certainly isn't me, or Tim Williamson. I'm guessing that the thing is an actual donkey, probably a rather bright one. Actually, I think this is one of those views that invites the incredulous stare. (Barcan formulae).